An in vitro DNA replication system from yeast has been developed as a model system to investigate the mechanism of DNA replication in eukaryotes. Among the proteins identified as essential for DNA replication by this system are yeast DNA polymerase I and a single-strained DNA binding protein. Recent biochemical fractionation of this system permitted the purification and characterization of a DNA primase activity. The potential role of DNA methylation in replication was investigated by measuring the extent of methylation of 2Mum and ars-1 containing plasmid DNAs at different stages of the cell division cycle and by comparing as templates in the in vitro DNA replication system plasmid DNAs which were unmethylated or methylated by various methylases.